The invention generally relates to methods of triacylglycerol oil refining and is based on using flow-through hydrodynamic cavitation. The invention utilizes energy released upon the implosion of cavitation bubbles to purify oils and improve the commercial value of collected by-products. More particularly, the present invention relates to lowering the levels of sterol glucosides (SGs) and acylated sterol glucosides (ASGs) and enzyme-hydrolyzable phospholipids which can be followed by biodiesel production through transesterification. The residual concentrates obtained from the invention can be used as blood cholesterol lowering food additives, in pharmaceuticals' production or for other purposes. The invention finds application in biofuel, chemical, food, pharmaceutical and other industries.
Crude vegetable oils are comprised mostly of triacylglycerols (TAG) and contain impurities such as phospholipids (phosphatides), free fatty acids (FFA), off-flavor compounds, carotenes, chlorophyll and other pigments, waxes, aluminum, calcium, copper, iron, magnesium and other metals and phytosterols. The impurities negatively affect the quality of oil and oil-derived products and must be removed before use.
The crude oil can be produced by solvent extraction or by pressing seeds either with heating or without it. The hot pressing affords the better yield but results in oil deterioration and the accumulation of non-hydratable phosphatides (NHP), for example calcium and magnesium salts of phosphatidic acid (PA) and phosphatidyl ethanolamine (PE) due to the action of enzymes that are active at 57-85° C. PE can be hydrated if it has a net charge. PA has a glycerol backbone usually with a saturated fatty acid, an unsaturated acid, and a phosphate group attached to carbon 1, 2 and 3, correspondingly. To assure a high quality of oil, oil producers avoid exposing seed to temperatures around 55° C.-80° C. and treat them with steam at approximately 150° C. to deactivate phospholipases and lower PA salt level by 25-50% (Cmolik and Pokorny, 2000; Gunstone et al., 2007).
Oil refining methods depend on the type of oil and usually comprises degumming, bleaching and deodorization. Degumming is the removal of phosphorus present in the form of hydratable and non-hydratable phosphatides. Water degumming provides refined oil with a phosphorus concentration greater than 200 ppm and can be followed by alkali refining, bleaching and deodorizing or by acid degumming, dry degumming and physical refining or by enzymatic degumming (Clausen, 2001), bleaching and physical refining. There are numerous variations of oil refining methods, depending on the quality of oil and other conditions. In addition, oil can be hydrogenated to afford a stable product.
Each refining step results in some loss of oil. (Racicot and Handel, 1982; Cvengros, 1995; Cmolik and Pokorny, 2000) The oil yield can be increased by using enzymes instead of chemical reagents. For example, phospholipase C hydrolyzes phosphatidylcholine (PC), liberating the water-soluble phosphate ester of choline and diacylglycerol (DAG). The conversion of phospholipids to DAG increases the oil yield due to the accumulation of DAG in the oil phase and minimal entrapment of neutral oil in gums comprised of hydrated lecithin. PC is converted by phospholipases A1 and A2 to lysophosphatidylcholine and FFA. Lipid acyltransferase (LAT) catalyzes PC breakdown to lysophosphatidylcholine and FFA, which can form esters with the free sterols present in oil. Accordingly, PE is converted by phospholipases A1 and A2 and LAT to lysophosphatidylethanolamine (LPE) and FFA or steryl esters. LPE is a plant growth regulator that can be isolated as a valuable by-product. Phospholipase C catalyses the hydrolysis of PE to ethanolamine-phosphate and DAG. Phosphatidylinositol (PI) can be hydrated over a wide pH range and is converted by phospholipases A1 and A2 and LAT to lysophosphatidylinositol. However, PI is not hydrolyzed by phospholipase C. Phospholipases A1 and A2 and LAT convert alkali salts of PA to lysophosphatidic acid salts. Alkali salts of PA are not affected by phospholipase C.
Since phospholipases A1 and A2 and LAT are soluble in water, they act on the phosphatides located at the oil/water interface. As a consequence, the enzymatic degumming requires long-duration, high-shear agitation to sustain the large oil/water surface area and high mass transfer rates and slows down with the coalescence of water-in-oil dispersion. Oil producers do not use emulsifiers for the stabilization of dispersions on an industrial scale because of their high cost.
SGs are sterol derivatives, in which a carbohydrate unit (arabinose, glucose, etc.) is linked to the hydroxyl group of campesterol, brassicasterol, dihydrositosterol, sitosterol, stigmasterol or other sterols with an ether bond. In ASGs, which are very soluble in vegetable oils, the carbohydrate 6-carbon is esterified with a long chain fatty acid. Phytosterols are abundant in plants and can be readily isolated. (Sugawara and Miyazawa, 1999) They are cellular stress mediators and possess anticancer properties. SGs were reported to exhibit a neurotoxic effect and are a potential causal factor in the motor neuron pathology previously associated with cycad consumption and amyotrophic lateral sclerosis-parkinsonism dementia complex. (Khabazian et al., 2002; Ly et al., 2006; Bradford and Awad, 2007; Tabata et al., 2008) SGs are not soluble in biodiesel or diesel and, therefore, cannot be forced through a diesel engine filter, resulting in a clogged fuel system. SG crystallizes at about 35 ppm at room temperature leading to the formation of haze in biodiesel. SGs and ASGs melt at approximately 240 and 250-300° C. and promote the crystallization of other compounds present in biodiesel at cold temperatures by becoming the seed crystals for large agglomerates. Thus, it is necessary to lower the ASG and SG content of oil feedstock prior to the production of biodiesel.
The level of ASG and SG in biodiesel drops as a result of biodiesel storage due to the sedimentation of agglomerates. ASG can be converted to SG during the base-catalyzed transesterification, for example in alkali-catalyzed methanolysis. (Lepage, 1964) The acid hydrolysis of both SG and ASG liberates the corresponding free sterols, which are not soluble in biodiesel. LAT catalyzes conversion of free sterols to steryl esters.
Crude palm, soybean, corn and sunflower oil scan contain up to 2,500, 2,300, 500 and 300 ppm SGs, respectively. The SG content of palm and soybean biodiesel is 55-275 and 0-158 ppm, correspondingly. (Van Hoed et al., 2008) To evaluate biodiesel contamination level and filterability, ASTM D2068-08 “Standard Test Method for Determining Filter Blocking Tendency” and ASTM D6751-09a “Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels” are used. The Filter Blocking Tendency (FBT) value of soybean biodiesel with ˜70 ppm SG is approximately fifteen. The value for FBT of diatomaceous earth-filtered biodiesel with ˜20 ppm SG is close to one. The sticky residue retained with filters at palm or soybean biodiesel plants contains up to 50 and 25% of SG and ASG, correspondingly. SGs exhibit high adsorption capacity towards fatty acid methyl esters which results in their entrapment. (Van Hoed et al., 2008)
The purification of oil prior to biodiesel production lowers both phosphorus and phytosterol concentration in the final product. Although SGs can be removed by using filtration, absorption or distillation (Manjula and Subramanian, 2006; Bondioli et al., 2008), biodiesel manufacturers are especially interested in the development of a cost-effective, high-throughput method that lowers the levels of phosphorus, SGs and ASGs in oil feedstock and allows the recovery of valuable residual concentrates.
Most vegetable oils can be purified in the accordance with the present invention including acai, almond, arachis, avocado, buckthorn, camelina, candlenut, canola, cashew, castor, citrus, cocoa butter, coconut, corn, cottonseed, evening primrose, grape seed, groundnut, hazelnut, hemp, jojoba, linseed, macadamia, meadowfoam seed, mongongo, mustard, ojon, olive, palm, papaya, peanut, pecan, pine nut, pistachio, poppyseed, radish, rapeseed, rice bran, safflower, sesame, soybean, sunflower, tung, and walnut oils. The invention is also applicable to algal oil, animal fat, bird fat, fish fat, tallow and grease.
It is known that the increase in both pressure and temperature and the vigorous mixing provided by cavitation can initiate and/or accelerate chemical reactions and processes. Although extreme conditions can be disadvantageous, the outcome of an optimized controlled cavitation treatment is always beneficial. Therefore, the reaction yield enhancement by means of the energy released upon the collapse of generated cavitation bubbles has found a number of applications.
Cavitation can be hydrodynamic, acoustic, ultrasonic, light irradiation-induced, steam injection-generated, etc. Simultaneous application of cavitation-generating methods improves the efficiency (Moulton and Mounts, 1999; Young, 1999; Gogate, 2008; Mahulkar et al., 2008).
If fluid flow is directed in a flow-through hydrodynamic cavitation apparatus at a proper velocity, the vapor-filled bubbles will form within the flow due to the drop in hydrolytic pressure. The bubbles collapse in a slow-velocity, high-pressure zone, causing sharp increases in both pressure and temperature, the formation of high-velocity streams and shock waves, vigorous shearing forces, and the release of a substantial amount of energy. This process activates atoms, molecules, ions and/or radicals located in the bubbles and the surrounding liquid, and initiates chemical reactions and processes. The bubble implosion can also result in the emission of light favoring photoreactions and radical generation.
The cavitation phenomenon is categorized by cavitation number Cv, defined as: Cv=(P−Pv)/0.5 ρV2, where P is the pressure downstream of a constriction, Pv is the fluid's vapor pressure, ρ is the fluid's density, and V is the fluid's velocity at the orifice. Cavitation starts at Cv=1, and Cv<1 implies a high degree of cavitation. The number of cavitation events in a flow unit is another important parameter. (Suslick, 1989; Didenko et al., 1999; Suslick et al., 1999; Young, 1999; Gogate, 2008; Passandideh-Fard and Roohi, 2008; Zhang et al., 2008) Numerous flow-through hydrodynamic apparatuses are known. See, for example, U.S. Pat. No. 6,705,396 to Ivannikov et al., U.S. Pat. No. 7,338,551 to Kozyuk and U.S. Pat. No. 7,762,715 to Gordon et al.
With the cost of energy and human health concerns rising rapidly, it is highly desirable to develop a low-cost, environmentally friendly technology for the removal of phospholipids, SGs and ASGs from oils. To achieve as large profit margin as possible, it is necessary to decrease the time, energy consumption and oil loss during refining. The prior art methods do not offer the most efficient technologies for purifying oils in the shortest amount of time possible. As a result, the demand exists for an advanced method for the prompt removal of phytosterols and phospholipids from oil at low energy and agent cost resulting in products with advanced qualities, preferably using the flow-through cavitation. The present invention provides such method while delivering purified oil within a very short processing time. No accumulation of waste material harmful to the environment occurs, and the produced residual concentrates are suitable for downstream processing.
The invention provides an oil purification method based on generating cavitation in an oil flow within at least one cavitation apparatus' chamber, preferably in a number of the consecutively placed chambers. This goal is achieved through the application of cavitation apparatuses aimed at the express purification of oils. In accordance with the present invention, the method comprises feeding a fluidic mixture of oil and agent in the flow-through hydrodynamic cavitation device using a preset inlet pressure sustained by a pump and applying selected conditions and additional agents, if required.